1. Field of the Invention
The present invention relates to an image-pickup element capable of detecting a focus state of an image-pickup lens, and an image-pickup apparatus and a camera system having the same.
2. Description of the Related Art
Conventionally, some focus detection methods used in a digital camera which is an image-pickup apparatus are a contrast detection method used in a video camera or a compact camera, a phase difference detection method used in a single-lens reflex camera and the likes.
The single-lens reflex camera includes a flip-up mirror for guiding an object light into a finder optical system in order that a photographer observes the object through an image-pickup lens. The flip-up mirror is configured by a half mirror which transmits a part of light, and the light which has passed through the flip-up mirror is guided into a focus detection apparatus for a phase difference method in order to perform a focus detection of the image-pickup lens. The focus detection apparatus is configured by a group of lenses and a group of imaging sensors that re-images a light flux passing through different pupil areas of the image-pickup lens. When an image taking using a memory is performed, the flip-up mirror is retracted from an image-pickup optical path. Therefore, the focus detection in a conventional phase difference detection method cannot be performed while the image taking using the memory, such a film and an image-pickup element, is performed.
When a moving image is observed or stored in a camera including the image-pickup element as the memory, a focusing method in a contrast detection method is generally used. The focusing method in the contrast detection method extracts a high frequency component of an image taken by the image-pickup element while a focus lens of the image-pickup lens is moved in an optical axis direction, and performs the focusing by detecting a lens position at which the contrast becomes peak. This focusing method cannot perform the focusing with high speed because contrasts of the images are compared while the focus lens is moved.
To solve the above problems, this applicant discloses in Japanese Laid-Open No. 2010-117679 a camera that limits a light-receivable pupil area of the image-pickup lens by limiting a photoelectrical conversion area of a part of the focus detection pixels, which form a CMOS image-pickup element, and performs a focus detection by the phase difference method. In the Japanese Laid-Open No. 2010-117679, a microlens that forms a part of the focus detection pixel is configured so that a pupil of the image-pickup lens and a wire electrode have a conjugate relationship. As a result, a light flux that enters a photoelectrical conversion part is shield by the wire electrode, and thereby the light-receivable pupil area of the image-pickup lens is limited.
However, a microlens that forms the image-pickup element requires to be configured so that the pupil of the image-pickup lens and the photoelectrical conversion part substantially has a conjugate relationship. Therefore, this applicant discloses in Japanese Laid-Open No. 2009-244854 an image-pickup element that limits the light-receivable pupil area of the image-pickup lens by providing a light shielding part on the photoelectrical conversion part instead of light-shielding by the wire electrode that is disposed at a position distant from the photoelectrical conversion part. A metallic material, such a tungsten, is used in the light shielding part.
In a part of area on the photoelectrical conversion part of the CMOS image-pickup element, a transfer electrode is formed of polysilicon. When the polysilicon as the transfer electrode is covered from above in the tungsten as light shielding part, the height of a sensor increases and the light-receiving efficient of the CMOS image-pickup element (light-receiving angle characteristic or Fno proportionality) lowers.
For these problems, to shorten a distance from the microlens to the photoelectrical conversion part (that is to say, to decrease the height of the sensor), it is preferable to form the light shielding part formed in the focus detection pixel with similar thickness so as not to overlap the transfer electrode.
Therefore, this inventor invents a configuration that arrays the light shielding part and the transfer electrode on the photoelectrical conversion part in a direction perpendicular to an optical axis direction so that the light shielding part does not overlap the transfer electrode in the optical axis direction. In this case, the light shielding part and the transfer electrode can be arranged so as not to overlap each other in the optical axis direction, and the distance from the microlens to the photoelectrical conversion part can be shorten, thereby increasing the light-receiving efficient of the CMOS image-pickup element.
However, in this case, the polysilicon forming the transfer electrode is not covered in the tungsten forming the light shielding part, and is exposed to a light having passed through the image-pickup lens. The spectral transmission characteristics of the polysilicon forming the transfer electrode indicates a characteristic in that a short wavelength hardly transmits in blue side, but the transmittance increases as the wavelength increases and approaches a red side. Therefore, this case has a disadvantageous in that the focus detection cannot be performed with high accuracy because of the effect of a light passing the transfer electrode, depending on the position relationship between an open formed by the light shielding part and the transfer electrode.